This invention relates to a superconducting magnet apparatus and, more particularly, to a superconducting magnet apparatus with an emergency run-down system for use in an nuclear magnetic resonance imaging system.
FIG. 1. is a circuit diagram illustrating one example of a conventional superconducting magnet apparatus which is provided with an emergency run-down system such as disclosed in U.S. Pat. No. 4,807,084. As illustrated, the superconducting magnet apparatus comprises a superconducting main coil 1 for generating a substantially uniform static magnetic field in a region subject to the magnetic resonance imaging. Also, a pair of superconducting shim coils 2a and 2b is electromagnetically coupled to the superconducting main coil 1 for increasing the uniformity of the magnetic field generated by said superconducting main coil 1.
The superconducting magnet apparatus also comprises persistent current switches 3, 4a and 4b connected in parallel to the superconducting main coil 1 and the superconducting shim coils 2a and 2b. The persistent current switch 3 comprise a persistent current switch superconductor 5 and a persistent current switch heater 7 disposed in proximity to the superconductor 5. Similarly, the persistent current switches 4a and 4b comprise superconductors 6a and 6b and 8b, respectively. The superconducting magnet apparatus also comprises protective elements 9,10a and 10bwhich are resistors or diodes connected in parallel to the respective coil 9, 10a and 10b. The protective elements 9, 10a and 10b are provided for supressing the voltage generated across the persistent current switches 3, 4a and 4b upon the occurence of quenching at the superconductor main coil 1 and the superconductor shim coils 2a and 2b.
An emergency run down main coil heater element 11 is disposed in proximity of the superconducting main coil 1 for demagnetizing the main coil 1, and similar emergency run down shim coil heater elements 12a and 12b are disposed in proximity of the superconducting shim coils 2a and 2b for demagnetizing the shim coils 2a and 2b. The components thus far described are enclosed within a cryogenic vessel 13. The main coil heater element 11 and the shim coil heater elements 12a and 12b are connected to a heater power source 14 through a switch 15. The heater elements 12a and 12b are connected in series to each other but are connected in parallel to the main coil heater element 11.
If it is desired to rapidly demagnetize the superconducting magnet apparatus during its operation in the persistent current mode for any reason such as when an iron piece is attracted to the superconducting magnet or breaking out of a fire, the heater switch 15 is closed to energize and heat the main coil heater element 11 as well as the shim coil heater elements 12a and 12b by the electric power from the heater power source 14 to heat the superconducting main coil 1 and the superconducting shim coils 2a and 2b. As the temperature of the main coil 1 and the shim coil 2a and 2b exceed the respective predetermined critical temperature, the coils 1, 2a and 2b revert from the superconducting state to the normal conduction state. Therefore, a current circulating through a closed circuit composed of the superconducting main coil 1 and the persistent current switch 3, a circulating current through a closed circuit composed of the superconducting shim coil 2a and the persistent current switch 4a and a circulating current through a sclosed circuit composed of the superconducting shim coil 2b and the persistent current switch 4b rapidly decrease, thereby demagnetizing the superconducting main coil 1 and the superconducting shim coils 2a and 2b.
However, the critical temperature, at which a superconducting coil revert from the superconducting state to the normal state, generally decreases as the magnetic field and the current flowing therethrough increase, and the superconducting main coil 1 generally has the critical temperature lower than that of the superconducting shim coil 2a or 2b. Therefore, when the demagnetizing heater elements 11, 12a and 12b are concurrently initiated to be energized, the superconducting main coil 1 which has the lower critical temperature is brought into the normal conduction state and demagnetized before the superconducting shim coils 2a and 2b are demagnetized in the normal state. During the rapid decreasing of the current in the superconducting main coil 1 when it is demagnetized, a large electoromotive force and therefore and excessively large current is induced in the superconducting shim coils 2a and 2b which are still in the superconducting state.
This large current induced in the superconducting shim coils 2a and 2b disadvantageously generates severe mechanical and thermal stresses on the superconducting shim coils 2a and 2b. Also, a large capacity protective element is required for protecting each superconducting shim coil against an excessive voltage across the shim coil.